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This article was published in 1957
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INSTITUTE OF INSPECTORS OF STOCK OF N.S.W. YEAR BOOK.
Dieldrin, Aldrin and Diazinon
Use for the Control of Lice, Keds and Blowflies
G. C. BRANDER, B.Sc., M.R.C.V.S., Technical Director, William Cooper & Nephews (Aust.) Pty. Ltd.
In any consideration of the use of these newer insecticides it is important to mention the older ones still in use, and to explain why the newer insecticides should in time replace them. The older insecticides (arsenic, sulphur and derris) have been used for a very long time, and it is a tribute to their value that, in the case of arsenic and sulphur, they have been used for over 100 years. Arsenic alone, and arsenic-derris, give control of lice and lice and keds respectively. However, in the pastoral areas of the world where it is the practice to dip only once in the year, the residual action of these two materials is often insufficient to ensure 100 per cent. control of parasites. It is because of this that one welcomes the introduction of the insecticides, as in addition to the fact that they have a low toxicity in dipping baths, they have several very marked advantages.
Dieldrin, Aldrin and Diazinon have been found to control both lice and keds at very low levels indeed, and in the case of Dieldrin experiments have shown that it will control both lice and keds at concentrations as low as 0.0005% Dieldrin in a dip bath. This finding is obviously of great importance as it ensures that even in the hands of the careless user, Dieldrin when used for lice and ked control should always ensure complete elimination of parasites. At this low level, also, it had the ability to remain present in the fleece for several months; so that the sheep, in addition to being cleared of keds and lice at dipping, are protected from re-infestation exposure for several months. This factor is of particular value where re-mustering is not always carried out, and secondly where one grazier adjoins another who does not practice dipping thoroughly.
When considering the new insecticides it is thought of value to discuss how these insecticides are tested and how they can be used with confidence by the grazier.
Each new insecticide has to be screened to assess its value in the control of a particular group of insects. Before any tests can be carried out, however, it is essential that the chemical behaviour of the new insecticide be determined. If it is insoluble in water, it will have to be prepared in the form of a dust, wettable powder or a miscible oil.
It is usual to carry out the first formulation tests on the parasites of such small animals as guinea pigs or rabbits. At the same time tests will be carried out on the pests which do not live on animals; such as the house fly, grain weevil, cockroach,etc. As a result of these tests it is often possible to obtain a guide as to the likely concentration which is to be efficient against the parasites of animals.
In the first trials on small animals various concentrations are used; while the animals are treated by dipping or spraying, to simulate the type of treatment that would be used for larger animals. It is usual to use a wide range of concentrations so that any likely toxic action is determined as soon as possible. In the course of toxicity trials, numbers of animals in each group are killed to discover whether any tissue damage has occurred as a result of the treatment. It is advisable also to carry out serial treatments, so that should the insecticide be of a type that produces toxic reaction only after prolonged use, the required information is obtained.
After the likely concentration of insecticide has been determined, and also the likely toxic risk, it is usual to commence trials on the domestic animals such as sheep, cattle, poultry and pigs. As these animals are expensive it is the practice to test the initial concentration on small numbers of animals.
Biological tests are carried out on animals which are infested with lice and keds, and are susceptible to blowfly attack. In the course of these trials, animals are treated with four or five times the likely field dose, so that the toxic risks in the field can be assessed.
When efficient concentrations have been determined, it is usual to carry out field trials in which animals are treated under field conditions in various ways. In the case of sheep they would be treated in sheep showers, sheep dips and by spraying. It is considered correct to say that as far as the organisation with which the writer is associated is concerned, it is unusual for an insecticide to be offered for marketing until two or three years' field and laboratory trials have been carried out in this way it is possible for the technical side of the company to feel confident that usage will only verify the results that they have obtained in field trials.
The simplest way to discuss the advances in sheep parasite control is to consider each group of parasites separately; as frequently their control may involve separate investigations.
SHEEP BLOWFLY
Before discussing the value of these new insecticides for the control of the sheep blowfly it is of value to consider the various methods that are used to assess the activity of the new insecticides. When using D.D.T. as an insecticide for the control of blowfly its action was assessed mainly against the adult fly, as it had little action against the larval stages. It is usual, therefore, when testing anti-adult insecticides, to expose groups of sheep to masses of fed blowfly, which are attracted to the experimental sheep; these sheep having been treated with an attractant (a liver or indole concoction) which ensures that gravid blowflies lay their eggs on the wool.
It was soon found in the case of Dieldrin and Aldrin that they possessed outstanding larvicidal properties, and that they were also remarkably persistent in the fleece. Tests of their efficiency had therefore to be against the larval stages.
Du Toit and Fielder (1953) in South Africa, found that when they used Dieldrin and Aldrin they had a property which had not been encountered with previous insecticides; that of diffusing down the fleece.
There are two accepted methods of carrying out trials with insecticides to determine their value in the control of blowfly strike; these being larval implant tests and field trials. The larval implant method was first developed by McLeod. The method consisted of implanting first-stage blowfly larvae on to the skin of the test sheep. The larvae were kept moist by the addition of a cotton wool plug, and in later experiments the skin was scarified to assist the penetration of the larvae. It was found that this type of test gave a very good picture of the efficiency of both arsenic and derris as larvicides. With the development of Dieldrin and Aldrin it became necessary to carry out preliminary trials using this larval implant method. In experiments in Australia and England, it was found that both Dieldrin and Aldrin gave remarkably long periods of protection, even at relatively low concentrations.
In addition to these larval implant tests, field trials were carried out to test the efficiency of the insecticides in the prevention of both body and crutch strike. Field trials are always difficult to assess, and also difficult to carry out with complete success; since they are dependent on weather conditions, the thoroughness of the grazier, and the observer being in contact with the property at the time of strike waves.
However, both Dieldrin and Aldrin lived up to the picture they presented in the larval implant tests. Of the two insecticides, Dieldrin appears to give a longer period of protection than Aldrin, and also stands up to severe blowfly waves better than Aldrin.
Diazinon has been tested in Australia both by larval implant tests and field tests. In larval implant tests it gave even better results than Dieldrin or Aldrin; while subsequent field trials have suggested that tts efficiency is of a similar level to that of Dieldrin.
The essence of all treatment using these new insecticides is to ensure the treatment is carried out before a blowfly wave is expected. If this is done the grazier can be sure that sufficient insecticide will be present in the fleece to prevent the development of a strike. It is important, too, to differentiate between a real strike and an abortive strike, which occasionally occurs with these new insecticides. Where the strike is abortive a small area of larval penetration is noticed, but this never spreads and the larvae die out; usually within 24 hours.
Dieldrin and Aldrin now have been used in dipping baths for the control of blowfly, and Dieldrin in particular appears to give a good protection when used in this way. When used shortly after shearing, at a concentration of 0.025% Dieldrin, it will give two to three months protection from body strike, and about eight to ten weeks from crutch strike.
KEDS AND LICE
The initial trials using Dieldrin and Aldrin for the control of keds and lice showed that these insecticides were suited particularly for the control of these parasites, as they were effective at very low concentrations and also gave good residual protection. Again the level which had to be used was very much below the likely toxic level. Dieldrin will kill keds and body lice at concentrations as low as 0.0005%, and also will protect sheep from re-infestation for several months. As far as one knows no cases of toxic reaction associated with the insecticide has followed the use of Dieldrin or Aldrin in a dip.
DIAZINON
Trials with Diazinon have shown that this insecticide will control body lice and keds at similar concentration to Dieldrin, and that the residual effect obtained in the fleece will be similar to that when Dieldrin is used.
ITCH MITE
Although the itch mite has been reported from all parts of Australia, it is still difficult to obtain a true picture of the incidence of infestations. Little work has been reported since the early investigations carried out with lime-sulphur. Lime-sulphur will control itch mite, but unfortunately it has little effect on the other parasites, and in addition is unpleasant material to use. Therefore, the tendency has been to use the Arsenie and Arsenic Sulphur dips to control itch mite, at normal dipping time.
Experimental work using both Malathion and Diazinon have shown, under patch-test conditions, that both these insecticides show promise in the control of itch mite. Large-scale field trials should give us additional information.
METHODS OF APPLICATION
In any consideration of the new insecticides it is important to discuss the method of application. Although dipping has been used for many years and can give very satisfactory results, it has several disadvantages. Its success is dependent on the use of clean dip, and the correct calibration of the dipping tank. Unfortunately, under existing sheep management conditions, clean dips are not always possible, and in fact on some properties where large numbers of sheep have to be dipped, it is common practice for a dip to stand for periods of as long as a fortnight or three weeks. This practice exposes the last groups of sheep through the bath to the great danger of bacterial infections, and it is difficult to ensure that the right concentration of insecticide will be available for the last animals.
SHEEP SHOWERS
The sheep shower has been in use now for about fourteen years, and has proved itself as a method of sheep treatment. It has removed many of the disadvantages of the dipping tank; by providing reasonably clean wash, a sump which can be emptied after each day's treatment and a system of spray nozzles which do not clog and allow a deluge of wash to be applied to the sheep.
MOBILE SPRAY RACES
The property of diffusion along the wool fibres shown by Dieldrin has opened the way for the application of the new insecticides to sheep by means of a fine spray. It has been found that a light spray of two pints of 0.25% Dieldrin gives results in larval implant tests equivalent to those obtained with six pints of 0.05% Dieldrin applied as a dip. In these experiments the sheep were carrying eight weeks wool. Subsequent laboratory and field trials in Australia show that this method of application has considerable promise for off-shears treatment of sheep for lice. The spray is applied from 20 nozzles aimed to cover the whole body. The sheep run straight through the race, and the wash is not re-circulated. This type of race offers promise for use in pastoral areas where post-shearing mustering is difficult, and it can be taken to sheep which are isolated as a result of flooding or are too weak to be driven to the dip tank.
IN CONCLUSION
One would say that the insecticides for sheep treatment are available and can be formulated now for use in dips, showers or sprays. We must make sure that their use is fully understood by the sheepowner.